Fuel-cell vehicles using hydrogen as fuel emit no carbon dioxide (CO2) and have excellent energy efficiency, and so are expected to serve as vehicles that can solve CO2 emission problems and energy problems. To put such fuel-cell vehicles into wide use, hydrogen stations for supplying hydrogen to fuel-cell vehicles need to be installed. This has stimulated the development of vessels (pressure vessels) with excellent strength and durability necessary for safely storing high-pressure hydrogen in hydrogen stations.
Proposed pressure vessels using metal material include: a pressure vessel that is entirely made of metal (Type I); and a composite pressure vessel (Type II, III) formed by coating the outer periphery of a liner made of metal with carbon fiber reinforced plastic (CFRP).
For example, JP 2009-293799 A (PTL 1) proposes a composite pressure vessel whose fatigue crack growth rate in a high-pressure hydrogen environment is improved by coating the outer periphery of a liner made of Cr—Mo steel with CFRP. A pressure vessel made only of metal needs to be thick in order to have sufficient strength to withstand hydrogen pressure. In the composite pressure vessel described in PTL 1, on the other hand, the liner made of steel and the CFRP share the load, so that the liner can be made thinner than that of the pressure vessel made only of metal. This contributes to lighter weight and lower cost.
If the liner's share of load can be increased in the composite pressure vessel, the usage of expensive carbon fiber can be reduced, which further contributes to lower cost. This has raised the need to improve the properties of steel material used for composite pressure vessel liners.
To improve the properties of steel material used for pressure vessels, for example, JP 2010-037655 A (PTL 2), JP 2012-107332 A (PTL 3), JP 2009-275249 A (PTL 4), and JP 2009-074122 A (PTL 5) propose the following techniques. PTL 2 proposes steel material whose hydrogen embrittlement resistance is improved by controlling the chemical composition and microstructure of steel and the precipitates. PTL 3 proposes steel material whose toughness is improved by making the microstructure of steel composed mainly of bainite and controlling the aspect ratio of precipitated cementite. PTL 4 proposes steel material whose hydrogen embrittlement resistance is improved by controlling the chemical composition, thus achieving a high reduction of area in high-pressure hydrogen. PTL 5 proposes steel material whose hydrogen embrittlement resistance is improved by limiting the chemical composition of steel to a predetermined range and controlling carbide formation, thus achieving a high reduction of area in high-pressure hydrogen.